For example, a human head appears on images in various sizes and various shapes. Although a person can instantaneously and easily distinguish a human head from other items when seeing the human head with eyes, it is very difficult for a device to automatically distinguish the human head from other items. On the other hand, it is considered that the detection of a human head on images is important preprocessing and a fundamental technique in person detection. Particularly, in video image monitoring, there is a growing need for putting a technique capable of accurately detecting the human head to practical use as preprocessing of automatic and accurate person detection, person tracking, and measurement of a flow of people in various environments.
Regarding methods of detecting a human head, conventionally various methods are proposed (Patent Documents 1-4, and a Non-Patent Document 1). In these proposed detection methods, a circle or an ellipse is applied to a human head by various techniques on the assumption that the human head is basically circular or elliptic.
For example, the Patent Document 1 discloses a technique in which an ellipse is extracted by performing Hough transform vote to a brightness edge hierarchy image group produced from continuous two frame images by temporal difference and spatial difference, thereby detecting a person's head.
The Patent Document 2 discloses a technique in which a spatial distance image is produced from the video images taken by at least two cameras, an object is determined by dividing a region of the produced spatial distance image using a labeling technique, and circle fitting is applied to the determined object to obtain a person's head.
The Patent Document 3 discloses a technique in which the comparison is performed with not a simple ellipse template but a pattern (a part of the ellipse) as a reference pattern when the determination of the head is made, the pattern being obtained by decreasing intensity near a contact point with a tangential line perpendicular to an edge direction of an edge image.
The Patent Document 4 discloses a technique in which a head region that is a part of a foreground is estimated by computing a moment or a barycenter in a foreground region of a person extracted from an input image, and the ellipse applied to the person's head is determined based on a shape of the region.
In the Non-Patent Document 1, a technique is disclosed, in which a semicircle is found to seek a head candidate using the Hough transform, a profile probability of each point on a profile line is computed from the head candidate to determine whether or not the head candidate is a head.
In these techniques, a circle or an ellipse is searched from the image on the assumption that a person's head is basically the circle or the ellipse. However, basically it takes a long time to search the circle or the ellipse in the whole image region, and it is difficult to analyze multiple sequentially-transmitted images such as a video image in real time.
There has been proposed another technique in which a differential image is produced with a continuous image such as the video image to catch a human moving on the continuous image (See Patent Documents 5, 6).
FIG. 1 is an explanatory view of a method that produces a differential image from background images.
Part (A) of FIG. 1 shows a technique in which a differential image between two images of different frames, for example, an immediately preceding frame and a present frame are produced using a moving image including plural continuous frames and a moving object (for example, human) emerging on the differential image between the two frame images is extracted.
Part (B) of FIG. 1 shows a technique in which an average background image is produced from the plural frame images and a differential image between the average background image and the present frame image is produced to extract the moving object emerging on the differential image.
In the differential image producing techniques of FIG. 1, the whole outline of the human does not purely come to the differential image, but sometimes small parts of the human separately emerges on the differential image or sometimes the background except the human also partially emerges on the differential image due to a change of illumination. Therefore, there is a need for such an analysis and integration operation that the outline of the human is estimated while some patterns emerging on the differential image are combined or deleted. The moving object cannot be extracted unless the analysis and integration operation is accurately performed. On the other hand, when the analysis and integration operation is accurately performed, a long time is required, which results in difficulty of real-time analysis.    Patent Document 1: Japanese Patent Application Laid-Open No. 2004-295776    Patent Document 2: Japanese Patent Application Laid-Open No. 2005-92451    Patent Document 3: Japanese Patent Application Laid-Open No. 2005-25568    Patent Document 4: Japanese Patent Application Laid-Open No. 2007-164720    Patent Document 5: Japanese Patent Application Laid-Open No. 2007-323572    Patent Document 6: Japanese Patent Application Laid-Open No. 2006-318341    Non-Patent Document 1: “Real-time multiple head shape detection and tracking system with decentralized trackers”, ISDA, 2006, by Jacky. S. C. Yuk, Kwan-Yee K. Wong, Ronald H. Y. Chung, F. Y. L Chin, and K. P. Chow.